Suppose a well-known aunt is robbed of £50 (3)

I believe the answer is:

say

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'suppose' is the definition.
(I know that suppose can be written as say)

I cannot understand how the remainder of the clue works.

(Other definitions for say that I've seen before include "Judge" , "Express in words" , "Utter; e.g." , "Articulate" , "power" .)

I've seen this clue in the Evening Standard.

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